As exhaust gas purifying systems for purifying and discharging exhaust gas from diesel engines, those with a diesel particulate defuser (DPD) and a selective catalytic reduction (SCR) device attached to an exhaust pipe have been developed.
In this type of exhaust gas purifying system, the DPD captures PM in the exhaust gas. This type of exhaust gas purifying system is provided with an SCR system including the SCR device. In the SCR system, a urea solution stored in a urea tank is supplied into the exhaust gas upstream of the SCR to generate ammonia with heat of the exhaust gas. The ammonia is then used to reduce and purify NOx on the SCR catalyst (refer to, for example, Patent Documents 1 and 2).
PM captured by the DPD may clog a filter of the DPD. It is thus necessary to appropriately oxidize and remove PM captured and accumulated by the DPD to regenerate the DPD.
To detect clogging of the filter, an exhaust gas pressure sensor detects a differential pressure across the DPD. When the differential pressure reaches an upper limit, an engine control unit (ECU) automatically starts regeneration of the DPD. Alternatively, to start the regeneration in a manual manner, a DPD warning lamp in a cabin is illuminated to prompt a driver to press a regeneration start switch.
The DPD is provided with a diesel oxidation catalyst (“DOC”) and a catalyzed soot filter (CSF). The DOC includes an active catalyst for oxidizing unburned fuel. The CSF captures PM in the exhaust gas. During the regeneration of the DPD, multiple fuel injections are delivered (pilot injection, pre-injection, main injection, and after-injection) to increase an exhaust gas temperature to or above a catalyst active temperature of the DOC. Post injection is then added to increase the exhaust gas temperature to 500° C. or above, so that the obtained hot exhaust gas burns and removes PM captured by the CSF.
The DPD may be regenerated automatically while a vehicle is running or manually at an idle rpm while the vehicle is stopping. In general, the DPD is regenerated automatically with a vehicle running. Since the post injection causes fuel to mix into lubricant in a cylinder and thus dilute the lubricant, the manual regeneration is available to reduce the level of dilution.
During the automatic regeneration with a vehicle running, if the vehicle stops, an exhaust brake valve is designed to close in order to prevent an exhaust gas temperature from dropping. This is to allow the DPD to be regenerated at an idle rpm without interruption.
During the automatic regeneration, an exhaust gas temperature sensor disposed downstream of the DOC is used to detect the temperature of the exhaust gas flowing into the CSF. A deviation of the detected exhaust gas temperature from a target regeneration temperature is then evaluated. The deviation is used for proportional-integral-derivative (“PID”) control to adjust an amount of post injection so as to achieve the target regeneration temperature.